Attorney General Kamala Harris listens as former U.S. secretary of State George Shultz speaks at a Smart on Crime policy summit at U.C. Hastings College of the Law in San Francisco, Wednesday, March 16, 2011.

California Attorney General Kamala Harris vowed Thursday to redouble efforts to prevent Mexican drug cartels from committing crimes north of the border.

Harris and about 20 state, federal and local officials met in Imperial to discuss cross-border crime, and the state's top lawyer said she's adding four agents to Imperial County, bringing the total to 12.

The Imperial County office will be headed by Javier Salaiz, who led an investigation of the Tijuana, Mexico-based, Arellano Felix cartel's presence in California. Three men pleaded not guilty in San Diego last month to plotting to murder a Southern California couple suspected of owing money to the cartel.

Among the defendants in that case is Jorge Sillas, 28, whose older brother, Juan Sillas, has emerged as one of Tijuana's most violent hit men over the past two years. The elder Sillas - known as Ruedas, or Wheels in English - is suspected by authorities of being responsible for many of the border city's more than 800 murders last year.

"Imperial Valley is a very busy area," Salaiz said. "Any law enforcement entity in Imperial County is going to be a great asset."

Harris and other authorities also planned to visit the border city of Calexico to tour one of the dozens of cross-border tunnels discovered in recent years that are used to smuggle drugs. The secret passages are concentrated in California and near Nogales, Ariz. Two sophisticated tunnels that connected San Diego and Tijuana netted 50 tons of marijuana when they were discovered in November.

Imperial County has been the starting point for some of the U.S. Drug Enforcement Administration's highest-profile investigations in recent years. A 2005 traffic stop in the county seat of El Centro launched Operation Imperial Emperor. Its main target - high-level Sinaloa cartel operative Victor Emilio Cazares Gastellum - eluded capture but authorities announced a slew of arrests and seizures nationwide in 2007.

The farming region of 175,000 people is one of the main U.S. drug smuggling corridors. Imperial County District Attorney Gil Otero said it is a transit point to Los Angeles.

Seizures in Imperial County are measured in tons, not pounds, Otero told reporters.

"If we were to party every day - 24 hours a day, 365 days a year - we could never consume that much drugs in Imperial County," he said.

Harris said some violence from Mexico has spilled north of the border.

"Is that a reason for a high level of hysteria?" she told reporters. "No, but it is a reason to rededicate resources to the local, state and federal law enforcement agencies that are focused on making sure that violence does not spill over, because we certainly do not and will not allow the blood that has been shed on the streets of Mexico to spill over to California."